Heretofore, there is known a superconducting cable using superconducting wire material as a conductor, which material becomes a superconducting state at an ultralow temperature. The superconducting cable is expected to serve as a power cable capable of transmitting large current with low loss, and is being developed for practical use.
The superconducting cable has a structure in which a cable core is housed in a thermal insulation tube, and the cable core is constructed by laminating a former, a superconductive conductor layer, an electric insulating layer, etc. in order from the center of the cable core.
The thermal insulation tube has a double tube structure having an internal tube and an external tube, wherein vacuum evacuation is performed between the internal tube and the external tube, and a cooling medium such as liquid nitrogen circulates inside the internal tube so that electrical power is transmitted in an ultralow temperature state.
Because the above-described superconducting cable is made to have a predetermined length, it is necessary to connect ends of the cables to each other with an intermediate connecting part in the case that the superconducting cable needs to be laid over a distance longer than the length of the superconducting cable.
In the intermediate connecting part, a connecting technique for an oil impregnated insulation cable (oil-filled (OF) cable) having a similar insulated structure is applied to a cable core connection. Concretely, as illustrated in FIG. 17, in each superconducting cable to be connected, insulating paper is formed in a taper shape by being exposed in a penciling form (pencil-like shape), the insulating paper constituting an electrical insulating portion outside a superconductive conductor 101.
Then, after connecting superconductive conductors to each other, an electric insulating portion 103 is newly formed by winding new insulating paper between taper shape portions of one electric insulating portion 102 and the other electric insulating portion 102 which are exposed in the penciling form and face each other, further a reinforcing insulating portion 104 which reinforces the insulating portions is formed by winding insulating paper around the periphery of the electric insulating portion 103 so that the reinforcing insulating portion 104 has a larger diameter than that of the existing insulating portions 102, and thereby the intermediate connecting part is insulated (for example, see non-patent document 1).